Noise Measurement: dB, dBA, Leq and Sound Level Meters

Decibels: dB, dBA, dBC and dBZ

The decibel is a logarithmic scale of sound pressure referenced to 20 µPa, the threshold of hearing. Because it is logarithmic, +3 dB is a doubling of sound energy, +10 dB is ten times the energy and is heard as roughly twice as loud. Two equal sources together give +3 dB, not double the number.

• dB (unweighted): the raw level in each frequency band, used in spectra and in calculations
• dBA (A-weighting): the response curve of the human ear at moderate levels; the scale used in almost every environmental, occupational and nuisance limit in Thailand
• dBC (C-weighting): much flatter at low frequency; used for peak levels, for bass-heavy sources such as music venues and generators, and for hearing-protector selection
• dBZ (Z-weighting): flat, no weighting; used for spectral and advanced analysis

Use the weighting the regulation or specification names. A dBA figure quoted against a dBC limit, or the reverse, is the most common error in reports we are asked to review. For the difference between a 10 dB and a 10 dB(A) clause in a contract, see 10 dB or 10 dB(A)?.

Time weighting: Fast, Slow and Impulse

• Fast (F, 125 ms): follows short peaks; the default for most Thai environmental measurements and for Lmax
• Slow (S, 1 s): a smoothed reading that is easier to read by eye; used where a standard or notification specifies it
• Impulse (I): weights short impacts; largely historical and only used where a method still calls for it

A report must state the time weighting used. An Lmax taken with Slow can read several decibels below the same event taken with Fast, which is enough to turn a fail into a pass on paper.

Metrics over time: Leq, Lmax, L10/L90, SEL, Lden

• Leq (LAeq,T): the energy-average level over the measurement period T; the basis of nearly every exposure and environmental limit, including Thailand's 24-hour limit of 70 dBA
• Lmax / Lmin: the highest and lowest level in the period, used for events and for Thailand's 115 dBA maximum
• L10 / L50 / L90: statistical levels exceeded for 10, 50 and 90 % of the time; L90 is the conventional background level and is what Thailand's nuisance calculation is built on
• SEL (LAE): the energy of one event normalised to one second, for aircraft or train pass-bys
• Lden / DNL: day-evening-night levels with evening and night penalties, used in European and some airport assessments

Choose the period to suit the question: 5 or 15 minutes for a nuisance check, one hour for a plant survey, 24 hours or longer for an environmental limit or an EIA baseline. Thailand's limits are listed on the noise limits reference table.

Exposure dose and TWA for workplaces

For employees the question is not the level in the room but the dose a person receives over the shift. A personal dosimeter worn for the whole shift gives:

• Dose (%): the exposure received as a percentage of the allowed daily exposure
• TWA: the time-weighted average level for the shift, normally 8 hours, compared with the limit in the Ministerial Regulation B.E. 2549
• The report must state the criterion level, the exchange rate and the threshold used; Thailand's regulation is built on a 90 dBA criterion with a 5 dB exchange rate, and a hearing conservation programme is required from an 8-hour TWA of 85 dBA

Geonoise runs these surveys as part of a hearing conservation programme, measured under real production and signed by a licensed noise analyst.

Instruments and accuracy classes

• Sound level meter: Class 1 to IEC 61672-1 is the instrument for legal and EIA work; Class 2 is for indicative surveys only
• Personal dosimeter: worn by the employee for dose and TWA
• Microphone: condenser or electret, always with a windscreen outdoors
• Acoustic calibrator: checked before and after every measurement, with the deviation recorded in the report
• Data logger or analyser: time history, statistics and spectra, for anything that has to be re-examined later

The instrument itself must carry a current calibration certificate from an ISO/IEC 17025 accredited laboratory. A phone app or an uncalibrated meter produces a number, not evidence.

Field practice and uncertainty

• Microphone position: the distance from walls, floor and ceiling the method requires, to limit reflections; 1.2–1.5 m height and 3.5 m from reflecting surfaces for community measurements
• Weather: no measurement in rain, wind speed below the method's limit, conditions logged
• Measurement period: covering both the peak of the activity and the quiet reference
• Source operating conditions: recorded, because a plant at half load is not the plant the complaint is about
• Uncertainty: instrument class, calibration, position and conditions all contribute, and the report should state them

Octave and one-third-octave spectra, tonality and low frequency

A single dBA number says how loud; a spectrum says where the energy is, which decides what will fix it.

• Octave and one-third-octave bands: locate the problem band and compare it with NC/NR or design curves
• Tonality: a prominent tone from a fan, motor or gear is penalised in several assessment methods and is usually the thing people complain about
• Low frequency: below 125 Hz, dBA understates the problem; mass, decoupling and sealing are the answer, not thin absorbent

From measurement to noise map

Measured levels, statistics and spectra feed a calculation model such as SoundPLAN, which turns a handful of positions into a contour map, lets options be compared before money is spent, and provides the before/after record once work is done. See noise modelling and prediction and noise monitoring.

Frequently asked questions